I actually read a study from a poultry science journal a few years back that tested ACV at different rates, and the results were pretty interesting. They found that anything over 2 tablespoons per gallon of water started showing some irritation in the chicks' crops after a few days. So your mileage may vary big time depending on how much you use. I've also heard from a few folks on a chicken forum I follow that it can mess with their electrolyte balance if you do it too often. My take is it's probably fine in small amounts but I wouldn't make it a daily thing.